This invention relates to a continuous low pressure gas phase process for the production of solid particulate polymers during an exothermic polymerization reaction in a uniform diameter vertical fluidizing bed reactor system which process comprises feeding a polymerization catalyst and a gaseous stream containing a least one polymerizable monomer to a fluidized bed of polymer particles and removing the exothermic heat of reaction by indirect cooling means in the reactor and removing dry particulate polymer. Also, this invention relates to a uniform diameter vertical fluidized bed reactor system containing an indirect cooling means in the reactor.